. Recent publication of the only two prospective studies of the relation of organochlorine exposure to breast cancer demonstrates need for more research. The findings of the two prior studies conflict: only one found increased risk associated with DDE. The only race-specific study found a protective association for poly-chlorinated biphenyls (PCBs) in Whites, and a suggestion of increase risk associated with PCBs among Blacks. The proposed study seeks to help resolve these discrepant findings. This prospective, nested case-control epidemiological study will be the first to investigate the relationship of exposure to 1,1-dichloro-2,2'-bis(p-chlorophenyl)-ethylene (DDE) and the PCBs during pregnancy to breast cancer incidence. The study population is a subset (N=275 case-control pairs) of the Child Health and Development Studies. Women enrolled during pregnancy from 1959-1967 and were followed for subsequent breast cancer by linkage to the California Cancer Registry. The study has the following aims: 1) assay frozen serum samples obtained during pregnancy for DDE and PCB; 2) test these hypotheses: a) organochlorine levels during pregnancy are associated with subsequent breast cancer' b) birth cohorts exposed to DDE prior to puberty show a stronger association between DDE and breast cancer than birth cohorts exposed after puberty; c) associations between organochlorine exposure and breast cancer are strongest among primigravidas; analyses in primigravidas may be most sensitive for detection of organochlorine effects because these analyses will capture both cumulative pubertal exposure and total exposure present prior to breast differentiation that follow first pregnancy; d) total PCBs are associated with increased breast cancer risk for Blacks and decreased risk for Whites; e) some PCB congeners are associated with decreased breast cancer risk while others are associated with increased risk; and f) race differences in the composition of total PCBs explain race differences in associations between total PCBs and breast cancer. The study will be the first to: 1) measure organochlorine exposure during pregnancy; and 2) consider timing of exposure in relation to puberty. Puberty and pregnancy are periods of increased vulnerability to carcinogenesis.